Can I copy a existing coins source code, edit it a bit, create a whitepaper and launch a ICO for my coin to collect essential funds for creating the phone app, wallet, marketing, etc.
I can't say I know a lot about it but since bitcoin is open source you definitely can do this and most of the original altcoins like litecoin, feathercoin, dogecoin and many others were like this. They were basically copies of bitcoin with slight modifications.
Exactly my plan. Only thing I want to change with the coin is the mining interface and mining speed. Have you heard of any coins that can be mined on your phone or something like that?
All coins can theoretically be mined on your phone... As long as the diff is really, really, really, really low.
When you use a hashing algorithm for which an ASIC exists, like sha256d, scrypt, x11,... the diff will skyrocket because of people trying to insta-mine your new coin, so it will be impossible to mine on a phone.
You should try to find an algo that can only be mined using a CPU, you might have a shot at mining with your phone then. Or even better, create your own algo.
That being said: mining on your phone is allmost never a good idear for any of the existing algorithms...